Passive communication systems are known, for example from international patent application publication number WO 90/07760 (Checkpoint Systems Inc.) and international patent application publication number WO 98/52142 (BTG International Ltd), in which the shape of the pulses is used to help distinguish signals from noise and interference. These systems are tag identification systems, where no particular problem of fraudulent interception is encountered.
A passive communication system is particularly, but not exclusively, applicable to an access control system of the kind including a portable transponder device that identifies the user of the device to a base station by transmitting a coded identification signal to the base station.
The base station may include a mechanism coupled to a door or other closure in order to unlock the door and enable physical entry of the user in response to a valid identification signal received from the portable transponder device, for example; however, the base station may be used alternatively or additionally to perform other types of access control, such as control of the enabling of an operational function, and in particular of starting an engine, or of the disabling of vehicle immobiliser functions, or again of the use of data processing equipment, for example.
The present invention is particularly, but not exclusively, applicable to communication by electromagnetic signals, which expression is to be understood as covering signals in which the magnetic field component is predominant as well as signals in which the magnetic and electric field components are of comparable magnitude.
In some known access control systems, the transponder device is of the active kind, in which an action of the user on the transponder device, such as pressing a button on the transponder device, is required to actuate the transmission of a signal over an electromagnetic transmission path to the base station. The necessity for such a voluntary action by the user is in itself some guarantee against fraudulent access. However, actuation of the device in this way by the user is additional to the actions associated with his primary purpose such as opening a door, starting an engine or starting use of the data processing equipment for example and accordingly is inconvenient.
The present invention relates to a communication system of the so-called ‘passive’ kind, that is to say of the kind comprising a transponder device capable of transmitting a coded identification signal or other data to the base station without specific action of the user on the transponder device in response to reception of an interrogation signal from the base station. If desired, the transponder device may also be provided with means that may be actuated by a user to trigger transmission to the base station without reception of an interrogation signal from the base station, to unlock access from a greater distance or to command locking, for example, in the case of an access control system. The interrogation by the base station may be triggered by an action of the user related to his primary purpose, such as pulling on a door handle to open a door, for example, and no further action by the user is then necessary to obtain access.
One problem that arises with passive access control systems is to prevent the signals exchanged between the base station and the transponder device being intercepted fraudulently by a ‘relay attack’. Thus, in the absence of precautions, when the user leaves the vicinity of the base station, a person could fraudulently trigger an interrogation by the base station, by pulling the handle of a door or actuating a push-button, for example; a first radio relay near to the base station could pick up the interrogation signal and transmit it to a second radio relay positioned by an accomplice near to the user; the second relay could pass the interrogation signal to the transponder device of the user and pick up a response from the transponder device without the user being aware of the interrogation and response; the second relay could then transmit the response to the first relay, which could then pass the response on to the base station.
Various approaches have been proposed to hinder or invalidate the fraudulent interception of communication in this way in passive access control systems, for example in international patent application specification WO 99/59284 and European patent specifications EP 01058214, EP 01041225 and EP 01001117. However, these proposals are not totally effective in making interception impossible, deteriorate the response time of the system and/or add complexity to the system.